| Background and Objectives:Cone-beam computed tomography(CBCT)mounted on a linear accelerator can generate volumetric images with the patient directly at the treatment position.CBCT has been used mainly for image-guided radiotherapy and adaptive radiotherapy(ART).3D-CBCT images acquired immediately before each treatment fraction provide exact anatomic information and can be used to recalculate dose distribution in order to track the dose to targets and OARs and trigger the re-planning,if properly calibrated.However,compared with planning FBCT images,CBCT images exhibit inferior image contrast and have more artifacts introduced by scatter and beam hardening.In order to improve the image quality,increase the accuracy of CBCT-based dose calculation and decompose materials on the molecular level,we proposed a set of dual energy imaging methods.Several experiments were done to evaluate the performance quantitatively.Materials and Methods:The X-ray volumetric imaging(XVI)system on a Synergy(Elekta,Stockholm,Sweden)system was used in this study.Graphite(C)and aluminum(Al)were selected as the basis materials for dual-energy imaging.The basis materials were scanned at a combination of different thicknesses using high and low energy X-rays to acquire the corresponding 2D projections(gH,gL)and then to create a look-up table(gH,gL)vs.(B1,B2).Then for the dual-energy imaging,two sets of CBCT projections were acquired with high and low energy X-rays,respectively.Each pair of high and low energy projections with an identical imaging angle was decomposed into two coefficient projections of C(B1)and Al(B2).Spatial distributions of coefficient for C(b1)and Al(b2)were obtained by reconstructing the three-dimensional images with the corresponding projections(B1 and B2),respectively.Then,the functional imaging,such as ED images,effective atomic number images and material composition images were calculated using b1,b2.The relationship between dual-energy imaging parameters and image quality was evaluated with the QCkV-1 phantom to choose the optimal imaging parameters.The imaging dose,image quality and the accuracy of dose calculation of dual-energy CBCT were measured and compared with conventional single-energy imaging.The accuracy of material decomposition with dual-energy imaging was also investigated experimentally.The imaging dose was measured with a standard dosimetry phantom using the volume CT dose index(CTZIvol).The image quality was evaluated quantitatively using a Catphan 503 phantom with the evaluation indices of the reproducibility of the RED values,high-contrast resolution(MTF50%),uniformity,and signal-to-noise ratio(SNR).Dose calculation of two simulated volumetric-modulated arc therapy plans using an Eclipse treatment-planning system(Varian Medical Systems,Palo Alto,CA,USA)was performed on an Alderson Rando Head and Neck(H&N)phantom and a Pelvis phantom.Fan-beam planning CT images for the H&N and Pelvis phantom were set as the reference.A global three-dimensional gamma analysis was used to compare dose distributions with the reference.The average gamma values for targets and OAR were analyzed with paired t-tests between DE-CBCT and SE-CBCT.A maderials decomposition method using the dual-energy imaging was proposed.It could decompose the object into water,lipid and protein.Its accuracy was evaluated with a specially designed phantom,which has nine plugs of different material composition.In addition,a linear mixed dual-energy imaging method was also proposed,which directly uses the image-domain 3D CBCT image to calculate the optimal linear coefficients of high and low energy images.The linear mixed images could correct beam hardening effect.The accuracy of the method was verified by using the Catphan phantom,the H&N and Pelvis phantoms.Results:The dual-energy imaging parameters of 70 kV&16 mAs and 120 kV&8-12.8 mAs has higher contrast noise ratio,lower noise and equivalent spatial resolution than other imaging parameters,and was selectee for dual-energy imaging.In two scans(H&N scan and body scan),the imaging dose of DE-CBCT increased by 1.0%and decreased by 1.3%.It had a better reproducibility of the RED values(mean bias:0.03 and 0.07)compared with SE-CBCT(mean bias:0.13 and 0.16).It also improved the image uniformity(57.5%and 30.1%)and SNR(9.7%and 2.3%),but did not affect the MTF50%.Gamma analyses of the 3D dose distribution with criteria of 1%/1 mm showed a pass rate of 99.0-100%and 85.3-97.6%for DE-CBCT and 73.5-99.1%and 80.4-92.7%for SE-CBCT.The average gamma values were reduced significantly by DE-CBCT(p<0.05).Gamma index maps showed that matching of the dose distribution between CBCT-based and reference was improved by DE-CBCT.As for materials decomposition,the mean bias was 1.4%,-0.8%and-0.6%for water,lipid and protein compositions respectively.The 95%limit of agreement was-2.1%to 5.9%,-3.9%to 2.2%and-3.3%to 2.1%for water,lipid and protein compositions respectively.For the linear mixed dual-energy imaging method,the correlation coefficients between the calculated RED and the reference were 0.995 and 0.975,respectively,which were better than SE-CBCT(0.975 and 0.953).Conclusions:DE-CBCT can achieve both better image quality and higher accuracy of dose calculation,and decompose the materias accurately.In conclusion,It could be applied to adaptive radiotherapy. |
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